Cellulose acetate films prepared by coating methods

ABSTRACT

A method of film fabrication is taught that uses a coating and drying apparatus to fabricate resin films suitable for optical applications. In particular, cellulose acetate films are prepared by simultaneous application of multiple liquid layers to a moving carrier substrate. After solvent removal, the cellulose acetate films are peeled from the sacrificial carrier substrate. Cellulose acetate films prepared by the current invention exhibit good dimensional stability and low birefringence.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Divisional of application Ser. No. 10/190,181, filed Jul. 3,2002 now U.S. Pat. No. 7,083,752, which is a 111A Application ofProvisional Application, Ser. No. 60/381,931, filed on May 20, 2002.

FIELD OF THE INVENTION

This invention relates generally to methods for manufacturing resinfilms and, more particularly, to an improved method for the manufactureof optical films, and most particularly, to the manufacture of celluloseacetate films used as substrates, compensation plates, and protectivecovers in optical devices such as light filters, liquid crystal displaysand other electronic displays.

BACKGROUND OF THE INVENTION

Cellulose acetate has historically been used as a support material inthe manufacture of photographic films. In particular, cellulose acetatefilms are known to have many desirable properties necessary for aphotographic film base including high transparency and relatively gooddimensional stability with respect to moisture and temperature.Cellulose acetate also has excellent resistance to degradation byultraviolet light and does not readily discolor when exposed to light orheat. These attributes have also made cellulose acetate films useful forthe fabrication of optical devices. For example, light polarizer platesutilize cellulose acetate films as covers to protect the delicatepolarizing film from distortion and damage. In this regard, celluloseacetate films have replaced glass to produce lightweight, flexiblepolarizer plates. Polarizers having cellulose acetate protective coversare utilized in liquid crystal displays, OLED (organic light emittingdiode) displays, and in other electronic displays found in, for example,personal computers, televisions, cell phones, and instrument panels.

Polymers of the cellulose acetate type are commercially available in avariety of molecular weights as well as the degree of acyl substitutionof the hyroxyl groups on the cellulose backbone. Of these, the fullysubstituted polymer, cellulose triacetate (CTA) having a combined aceticacid content of approximately 60% is commonly used to manufacturecellulose acetate films.

In general, resin films are prepared either by melt extrusion methods orby casting methods. Melt extrusion methods involve beating the resinuntil molten (approximate viscosity on the order of 100,000 cp), andthen applying the hot molten polymer to a highly polished metal band ordrum with an extrusion die, cooling the film, and finally peeling thefilm from the metal support. For many reasons, however, films preparedby melt extrusion are generally not suitable for optical applications.Principal among these is the fact that melt extruded films exhibit ahigh degree of optical birefringence. In the case of highly substitutedcellulose acetate, there is the additional problem of melting thepolymer. Cellulose triacetate has a very high melting temperature of270-300° C., and this is above the temperature where decompositionbegins. Films have been formed by melt extrusion at lower temperaturesby compounding cellulose acetate with various plasticizers as taught inU.S. Pat. No. 5,219,510 to Macbell. However, the polymers described inU.S. Pat. No. 5,219,510 to Machell are not the fully substitutedcellulose triacetate, but rather have a lesser degree of alkylsubstitution or have proprionate groups in place of acetate groups. Evenso, melt extruded films of cellulose acetate are known to exhibit poorflatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For thesereasons, melt extrusion methods are generally not practical forfabricating many resin films including cellulose triacetate films usedto prepare protective covers and substrates in electronic displays.Rather, casting methods are generally used to manufacture these films.

Resin films for optical applications are manufactured almost exclusivelyby casting methods. Casting methods involve first dissolving the polymerin an appropriate solvent to form a dope having a high viscosity on theorder of 50,000 cp, and then applying the viscous dope to a continuoushighly polished metal band or drum through an extrusion die, partiallydrying the wet film, peeling the partially dried film from the metalsupport, and conveying the partially dried film through an oven to morecompletely remove solvent from the film. Cast films typically have afinal dry thickness in the range of 40-200 microns. In general, thinfilms of less than 40 microns are very difficult to produce by castingmethods due to the fragility of wet film during the peeling and dryingprocesses. Films having a thickness of greater than 200 microns are alsoproblematic to manufacture due to difficulties associated with theremoval of solvent in the final drying step. Although the dissolutionand drying steps of the casting method add complexity and expense, castfilms generally have better optical properties when compared to filmsprepared by melt extrusion methods, and problems associated withdecomposition at high temperature are avoided.

Examples of optical films prepared by casting methods include: 1.)Cellulose acetate sheets used to prepare light polarizers as disclosedin U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289 to Caelas well as more recent disclosures in U.S. Patent Application. Serial.no. 2001/0039319 A1 to Harita and U. S. Patent Application. Serial.no.2002/001700 A1 to Sanefuji, 2.) Cellulose triacetate sheets used forprotective covers for light polarizers as disclosed in U.S. Pat. No.5,695,694 to Iwata, 3.) Polycarbonate sheets used for protective coversfor light polarizers or for retardation plates as disclosed in U.S. Pat.No. 5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 bothto Taketani, and 4.) Polyethersulfone sheets used for protective coversfor light polarizers or for retardation plates as disclosed in U.S. Pat.Nos. 5,759,449 and 5,958,305 both to Shiro.

Despite the wide use of the casting method to manufacture optical films,there are however, a number of disadvantages to casting technology. Onedisadvantage is that cast films have significant optical birefringence.Although films prepared by casting methods have lower birefringence whencompared to films prepared by melt extrusion methods, birefringenceremains objectionably high. For example, cellulose triacetate filmsprepared by casting methods exhibit in-plane retardation of 7 nanometers(nm) for light in the visible spectrum as disclosed in U.S. Pat. No.5,695,694 to Iwata. Polycarbonate films prepared by casting methodsexhibit in-plane retardation of 17 nm as disclosed in U.S. Pat. Nos.5,478,518 and 5,561,180 both to Taketani. U. S. Patent Application.Serial No. 2001/0039319 A1 to Harita claims that color irregularities instretched cellulose acetate sheets are reduced when the difference inretardation between widthwise positions within the film is less than 5nm in the original unstretched film. For many applications of opticalfilms, low in-plane retardation values are desirable. In particular,values of in-plane retardation of less than 10 nm are preferred.

Birefringence in cast films arises from orientation of polymers duringthe manufacturing operations. This molecular orientation causes indicesof refraction within the plane of the film to be measurably different.In-plane birefringence is the difference between these indices ofrefraction in perpendicular directions within the plane of the film. Theabsolute value of birefringence multiplied by the film thickness isdefined as in-plane retardation. Therefore, in-plane retardation is ameasure of molecular anisotropy within the plane of the film.

During the casting process, molecular orientation may arise from anumber of sources including shear of the dope in the die, shear of thedope by the metal support during application, shear of the partiallydried film during the peeling step, and shear of the free-standing filmduring conveyance through the final drying step. These shear forcesorient the polymer molecules and ultimately give rise to undesirablyhigh birefringence or retardation values. To minimize shear and obtainthe lowest birefringence films, casting processes are typically operatedat very low line speeds of 1-15 m/min as disclosed in U.S. Pat. No.5,695,694 to Iwata. Slower line speeds generally produce the highestquality films.

Another drawback to the casting method is the inability to accuratelyapply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward,conventional multi-slot casting dies create unacceptably non-uniformfilms. In particular, line and streak non-uniformity is greater than 5%with prior art devices. Acceptable two layer films may be prepared byemploying special die lip designs as taught in U.S. Pat. No. 5,256,357to Hayward, but the die designs are complex and may be impractical forapplying more than two layers simultaneously.

Another drawback to the casting method is the restrictions on theviscosity of the dope. In casting practice, the viscosity of dope is onthe order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Haywarddescribes practical casting examples using dopes with a viscosity of100,000 cp. In general, cast films prepared with lower viscosity dopesare known to produce non-uniform films as noted for example in U.S. Pat.No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowestviscosity dopes used to prepare casting samples are approximately 10,000cp. At these high viscosity values, however, casting dopes are difficultto filter and degas. While fibers and larger debris may be removed,softer materials such as polymer slugs are more difficult to filter atthe high pressures found in dope delivery systems. Particulate andbubble artifacts create conspicuous inclusion defects as well as streaksand may create substantial waste.

In addition, the casting method can be relatively inflexible withrespect to product changes. Because casting requires high viscositydopes, changing product formulations requires extensive down time forcleaning delivery systems to eliminate the possibility of contamination.Particularly problematic are formulation changes involving incompatiblepolymers and solvents. In fact, formulation changes are so timeconsuming and expensive with the casting method that most productionmachines are dedicated exclusively to producing only one film type.

Finally, cast films may exhibit undesirable cockle or wrinkles. Thinnerfilms are especially vulnerable to dimensional artifacts either duringthe peeling and drying steps of the casting process or during subsequenthandling of the film. In particular, the preparation of compositeoptical plates from resin films requires a lamination process involvingapplication of adhesives, pressure, and high temperatures. Very thinfilms are difficult to handle during this lamination process withoutwrinkling. In addition, many cast films may naturally become distortedover time due to the effects of moisture. For optical films, gooddimensional stability is necessary during storage as well as duringsubsequent fabrication of composite optical plates.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome thelimitations of prior art casting methods and provide a new coatingmethod for preparing amorphous cellulose acetate films having very lowin-plane birefringence.

It is a further object of the present invention to provide a new methodof producing highly uniform cellulose acetate films over a broad rangeof dry thicknesses.

Yet another object of the present invention is to provide a method ofpreparing cellulose acetate films by simultaneously applying multiplelayers to a moving substrate.

Still another object of the present invention is to provide a new methodof preparing cellulose acetate films with improved dimensional stabilityand handling ability by temporarily adhering the cellulose acetate filmto a supporting carrier substrate at least until it is substantially dryand then subsequently separating the carrier substrate from thecellulose acetate film.

Briefly stated, the foregoing and numerous other features, objects andadvantages of the present invention will become readily apparent uponreview of the detailed description, claims and drawings set forthherein. These features, objects and advantages are accomplished byapplying a low viscosity fluid containing cellulose acetate resin onto amoving carrier substrate by a coating method. The cellulose acetate filmis not separated from the carrier substrate until the coated film issubstantially dry (<10% residual solvent by weight). In fact, thecomposite structure of cellulose acetate film and carrier substrate maybe wound into rolls and stored until needed. Thus, the carrier substratecradles the cellulose acetate film and protects against shearing forcesduring conveyance through the drying process. Moreover, because thecellulose acetate film is dry and solid when it is finally peeled fromthe carrier substrate, there is no shear or orientation of polymerwithin the film due to the peeling process. As a result, celluloseacetate films prepared by the current invention are remarkably amorphousand exhibit very low in-plane birefringence.

Cellulose acetate films can be made with the method of the presentinvention having a thickness of about 1 to 500 μm. Very thin celluloseacetate films of less than 40 μm can be easily manufactured at linespeeds not possible with prior art methods. The fabrication of very thinfilms is facilitated by a carrier substrate that supports the wet filmthrough the drying process and eliminates the need to peel the film froma metal band or drum prior to a final drying step as required in thecasting methods described in prior art. Rather, the cellulose acetatefilm is substantially, if not completely, dried before separation fromthe carrier substrate. In all cases, dried cellulose acetate films havea residual solvent content of less than 10% by weight. Thus, the presentinvention readily allows for preparation of very delicate thin films notpossible with the prior art casting method. In addition, thick films ofgreater than 40 microns may also be prepared by the method of thepresent invention. To fabricate thicker films, additional coatings maybe applied over a film-substrate composite either in a tandem operationor in an offline process without comprising optical quality. In thisway, the method of the present invention overcomes the limitation ofsolvent removal during the preparation of thicker films since the firstapplied film is dry before application of a subsequent wet film. Thus,the present invention allows for a broader range of final film thicknessthan is possible with casting methods.

In the method of the present invention, cellulose acetate films arecreated by forming a single or, preferably, a multi-layer composite on aslide surface of a coating hopper, the multi-layer composite including abottom layer of low viscosity, one or more intermediate layers, and anoptional top layer containing a surfactant, flowing the multi-layercomposite down the slide surface and over a coating lip of the coatinghopper, and applying the multi-layer composite to a moving substrate. Inparticular, the use of the method of the present invention is shown toallow for application of several liquid layers having uniquecomposition. Coating aids and additives may be placed in specific layersto improve film performance or improve manufacturing robustness. Forexample, multi-layer application allows a surfactant to be placed in thetop spreading layer where needed rather than through out the entire wetfilm. In another example, the concentration of cellulose acetate in thelowermost layer may be adjusted to achieve low viscosity and facilitatehigh-speed application of the multi-layer composite onto the carriersubstrate. Therefore, the present invention provides an advantageousmethod for the fabrication of multiple layer composite films such asrequired for certain optical elements or other similar elements.

Wrinkling and cockle artifacts are minimized with the method of thepresent invention through the use of the carrier substrate. By providinga stiff backing for the cellulose acetate film, the carrier substrateminimizes dimensional distortion of the cellulose acetate film. This isparticularly advantageous for handling and processing very thin films ofless than about 40 microns. In addition, the restraining nature of thecarrier substrate also eliminates the tendency of cellulose acetatefilms to distort or cockle over time as a result of changes in moisturelevels. Thus, the method of the current invention insures that celluloseacetate films are dimensionally stable during preparation and storage aswell as during final handling steps necessary for fabrication of opticalelements.

In the practice of the method of the present invention it is preferredthat the substrate be a discontinuous sheet such as polyethyleneterephthalate (PET). The PET carrier substrate may be pretreated with asubbing layer or an electrical discharge device to modify adhesionbetween the cellulose acetate film and the PET substrate. In particular,a subbing layer or electrical discharge treatment may enhance theadhesion between the film and the substrate, but still allow the film tobe subsequently peeled away from the substrate.

Although the present invention is discussed herein with particularreference to a slide bead coating operation, those skilled in the artwill understand that the present invention can be advantageouslypracticed with other coating operations. For example, freestanding filmshaving low in-plane retardation should be achievable single or multiplelayer slot die bead coating operations and single or multiple layercurtain coating operations. Moreover, those skilled in the art willrecognize that the present invention can be advantageously practicedwith alternative carrier substrates. For example, peeling films havinglow in-plane birefringence should be achievable with other resinsupports (e.g. polyethylene naphthalate (PEN), cellulose acetate,polycarbonate, PET), paper supports, resin laminated paper supports, andmetal supports (e.g. aluminum). Practical applications of the presentinvention include the preparation of cellulose acetate sheets used foroptical films, polarizer plates, laminate films, release films,photographic films, and packaging films among others. In particular,cellulose acetate sheets prepared by the method of the present inventionmay be utilized as optical films in the manufacture of electronicdisplays such as liquid crystal displays. For example, liquid crystaldisplays are comprised of a number of film elements including polarizerplates, compensation plates and electrode substrates. Polarizer platesare typically a multi-layer composite structure having dichroic film(normally stretched polyvinyl alcohol treated with iodine) with eachsurface adhered to a protective cover. The cellulose acetate filmsprepared by the method of the present invention may are suitable asprotective covers for polarizer plates. The cellulose acetate filmsprepared by the method of the present invention may also be suitable forthe manufacture of compensation plates and electrode substrates.

The cellulose acetate film produced with the method of the presentinvention is an optical film. As produced the cellulose acetate filmmade with the method of the present invention will have a lighttransmittance of at least about 85 percent, preferably at least about 90percent, and most preferably, at least about 95 percent. Further, asproduced the cellulose acetate film will have a haze value of less than1.0 percent. In addition, the cellulose acetate films are smooth with asurface roughness average of less than 100 nm and most preferably with asurface roughness of less than 50 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary coating and drying apparatus thatcan be used in the practice of the method of the present invention.

FIG. 2 is a schematic of an exemplary coating and drying apparatus ofFIG. 1 including a station where the cellulose acetate web separatedfrom the substrate is separately wound.

FIG. 3 is a schematic of an exemplary multi-slot coating apparatus thatcan be used in the practice of the method of the present invention.

FIG. 4 shows a cross-sectional representation of a single-layercellulose acetate film partially peeled from a carrier substrate andformed by the method of the present invention.

FIG. 5 shows a cross-sectional representation of a single-layercellulose acetate film partially peeled from a carrier substrate andformed by the method of the present invention wherein the carriersubstrate has a subbing layer formed thereon.

FIG. 6 shows a cross-sectional representation of a multi-layer celluloseacetate film partially peeled from a carrier substrate and formed by themethod of the present invention.

FIG. 7 shows a cross-sectional representation of a multi-layer celluloseacetate film partially peeled from a carrier substrate and formed by themethod of the present invention wherein the carrier substrate has asubbing layer formed thereon.

FIG. 8 is a schematic of a casting apparatus as used in prior art tocast cellulose acetate films.

FIG. 9 is a bar graph illustrating the effect of different types ofsurfactants on the Mottle Index.

DETAILED DESCRIPTION OF THE INVENTION

Turning first to FIG. 1 there is shown a schematic of an exemplary andwell known coating and drying system 10 suitable for practicing themethod of the present invention. The coating and drying system 10 istypically used to apply very thin films to a moving substrate 12 and tosubsequently remove solvent in a dryer 14. A single coating apparatus 16is shown such that system 10 has only one coating application point andonly one dryer 14, but two or three (even as many as six) additionalcoating application points with corresponding drying sections are knownin the fabrication of composite thin films. The process of sequentialapplication and drying is known in the art as a tandem coatingoperation.

Coating and drying apparatus 10 includes an unwinding station 18 to feedthe moving substrate 12 around a back-up roller 20 where the coating isapplied by coating apparatus 16. The coated web 22 then proceeds throughthe dryer 14. In the practice of the method of the present invention thefinal dry film 24 comprising a cellulose acetate film on substrate 12 iswound into rolls at a wind-up station 26.

As depicted, an exemplary four-layer coating is applied to moving web12. Coating liquid for each layer is held in respective coating supplyvessel 28, 30, 32, 34. The coating liquid is delivered by pumps 36, 38,40, 42 from the coating supply vessels to the coating apparatus 16conduits 44, 46, 48, 50, respectively. In addition, coating and dryingsystem 10 may also include electrical discharge devices, such as coronaor glow discharge device 52, or polar charge assist device 54, to modifythe substrate 12 prior to application of the coating.

Turning next to FIG. 2 there is shown a schematic of the same exemplarycoating and drying system 10 depicted in FIG. 1 with an alternativewinding operation. Accordingly, the drawings are numbered identically upto the winding operation. In the practice of the method of the presentinvention the dry film 24 comprising a substrate (which may be a resinfilm, paper, resin coated paper or metal) with a cellulose acetatecoating applied thereto is taken between opposing rollers 56, 58. Thecellulose acetate film 60 is peeled from substrate 12 with the celluloseacetate film going to winding station 62 and the substrate 12 going towinding station 64. In a preferred embodiment of the present invention,polyethylene phthalate (PET) is used as the substrate 12. The substrate12 may be pretreated with a subbing layer to enhance adhesion of thecoated film 60 to the substrate 12.

The coating apparatus 16 used to deliver coating fluids to the movingsubstrate 12 may be a multi-layer applicator such as a slide beadhopper, as taught for example in U.S. Pat. No. 2,761,791 to Russell, ora slide curtain hopper, as taught by U.S. Pat. No. 3,508,947 to Hughes.Alternatively, the coating apparatus 16 may be a single layerapplicator, such as slot die bead hopper or jet hopper. In a preferredembodiment of the present invention, the application device 16 is amulti-layer slide bead hopper.

As mentioned above, coating and drying system 10 includes a dryer 14that will typically be a drying oven to remove solvent from the coatedfilm. An exemplary dryer 14 used in the practice of the method of thepresent invention includes a first drying section 66 followed by eightadditional drying sections 68-82 capable of independent control oftemperature and air flow. Although dryer 14 is shown as having nineindependent drying sections, drying ovens with fewer compartments arewell known and may be used to practice the method of the presentinvention. In a preferred embodiment of the present invention the dryer14 has at least two independent drying zones or sections.

Preferably, each of drying sections 68-82 each has independenttemperature and airflow controls. In each section, temperature may beadjusted between 5° C. and 150° C. To minimize drying defects from casehardening or skinning-over of the wet cellulose acetate film, optimumdrying rates are needed in the early sections of dryer 14. There are anumber of artifacts created when temperatures in the early drying zonesare inappropriate. For example, fogging or blush of cellulose acetatefilms is observed when the temperature in zones 66, 68 and 70 are set at25° C. This blush defect is particularly problematic when high vaporpressures solvents (methylene chloride and acetone) are used in thecoating fluids. Aggressively high temperatures of 95° C. in the earlydrying sections 66, 68, and 70 are found to cause premature delaminationof the cellulose acetate film from the carrier substrate. Highertemperatures in the early drying sections are also associated with otherartifacts such as case hardening, reticulation patterns and blisteringof the cellulose acetate film. In preferred embodiment of the presentinvention, the first drying section 66 is operated at a temperature ofat least about 25° C. but less than 95° C. with no direct airimpingement on the wet coating of the coated web 22. In anotherpreferred embodiment of the method of the present invention, dryingsections 68 and 70 are also operated at a temperature of at least about25° C. but less than 95° C. It is preferred that initial drying sections66, 68 be operated at temperatures between about 30° C. and about 60° C.It is most preferred that initial drying sections 66, 68 be operated attemperatures between about 30° C. and about 50° C. The actual dryingtemperature in drying sections 66, 68 may optimize empirically withinthese ranges by those skilled in the art.

Referring now to FIG. 3, a schematic of an exemplary coating apparatus16 is shown in detail. Coating apparatus 16, schematically shown in sideelevational cross-section, includes a front section 92, a second section94, a third section 96, a fourth section 98, and a back plate 100. Thereis an inlet 102 into second section 94 for supplying coating liquid tofirst metering slot 104 via pump 106 to thereby form a lowermost layer108. There is an inlet 110 into third section 96 for supplying coatingliquid to second metering slot 112 via pump 114 to form layer 116. Thereis an inlet 118 into fourth section 98 for supplying coating liquid tometering slot 120 via pump 122 to form layer 124. There is an inlet 126into back plate 100 for supplying coating liquid to metering slot 128via pump 130 to form layer 132. Each slot 104, 112, 120, 128 includes atransverse distribution cavity. Front section 92 includes an inclinedslide surface 134, and a coating lip 136. There is a second inclinedslide surface 138 at the top of second section 94. There is a thirdinclined slide surface 140 at the top of third section 96. There is afourth inclined slide surface 142 at the top of fourth section 98. Backplate 100 extends above inclined slide surface 142 to form a back landsurface 144. Residing adjacent the coating apparatus or hopper 16 is acoating backing roller 20 about which a web 12 is conveyed. Coatinglayers 108, 116, 124, 132 form a multi-layer composite which forms acoating bead 146 between lip 136 and substrate 12. Typically, thecoating hopper 16 is movable from a non-coating position toward thecoating backing roller 20 and into a coating position. Although coatingapparatus 16 is shown as having four metering slots, coating dies havinga larger number of metering slots (as many as nine or more) are wellknown and may be used to practice the method of the present invention.

In the method of the present invention, the coating fluids are comprisedprincipally of a cellulose ester dissolved in an organic solvent.Cellulose esters are commercially available in a variety of molecularweight sizes as well as in the type and degree of alkyl substitution ofthe hydroxyl groups on the cellulose backbone. Examples of celluloseesters include those having acetyl, proprionyl and butyryl groups. Ofparticular interest is the family of cellulose esters with acetylsubstitution known as cellulose acetate. Of these, the fully acetylsubstituted cellulose having a combined acetic acid content ofapproximately 58.0-62.5% is known as cellulose triacetate (CTA) and isgenerally preferred for preparing protective covers, compensation films,and substrates used in electronic displays.

In terms of organic solvents for cellulose acetate, suitable solvents,for example, include chlorinated solvents (methylene chloride and 1,2dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol,n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones(acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone),esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropylacetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate),aromatics (toluene and xylenes) and ethers (1,3-dioxolane,1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). In someapplications, small amounts of water may be used. Normally, CTAsolutions are prepared with a blend of the aforementioned solvents.Preferred primary solvents include methylene chloride, acetone, methylacetate, and 1,3-dioxolane. Preferred co-solvents include methanol,ethanol, n-butanol and water.

Coating fluids may also contain plasticizers. Appropriate plasticizersfor cellulose acetate films include phthalate esters (dimethylphthalate,diethylphthalate, dibutylphthalate, dioctylphthalate, didecylphthalateand butyl octylphthalate), adipate esters (dioctyl adipate), andphosphate esters (tricresyl phosphate and triphenyl phosphate).Plasticizers are normally used to improve the physical and mechanicalproperties of the final film. In particular, plasticizers are known toimprove the flexibility and dimensional stability of cellulose acetatefilms. However, plasticizers are also used here as coating aids in theconverting operation to minimize premature film solidification at thecoating hopper and to improve drying characteristics of the wet film. Inthe method of the present invention, plasticizers are used to minimizeblistering, curl and delamination of cellulose acetate films during thedrying operation. In a preferred embodiment of the present invention,plasticizers are added to the coating fluid at a total concentration ofup to 50% by weight relative to the concentration of polymer in order tomitigate defects in the final cellulose acetate film.

Coating fluids may also contain surfactants as coating aids to controlartifacts related to flow after coating. Artifacts created by flow aftercoating phenomena include mottle, repellencies, orange-peel (Bernardcells), and edge-withdraw. Surfactants used control flow after coatingartifacts include siloxane and fluorochemical compounds. Examples ofcommercially available surfactants of the siloxane type include: 1.)Polydimethylsiloxanes such as DC200 Fluid from Dow Corning, 2.)Poly(dimethyl, methylphenyl)siloxanes such as DC510 Fluid from DowCorning, and 3.) Polyalkyl substituted polydimethysiloxanes such asDC190 and DC1248 from Dow Coming as well as the L7000 Silwet series(L7000, L7001, L7004 and L7230) from Union Carbide, and 4.) Polyalkylsubstituted poly(dimethyl, methylphenyl)siloxanes such as SF1023 fromGeneral Electric. Examples of commercially available fluorochemicalsurfactants include: 1.) Fluorinated alkyl esters such as the Fluoradseries (FC430 and FC431) from the 3M Corporation, 2.) Fluorinatedpolyoxyethylene ethers such as the Zonyl series (FSN, FSN100, FSO,FSO100) from Du Pont, 3.) Acrylate:polyperfluoroalkyl ethylacrylatessuch as the F series (F270 and F600) from NOF Corporation, and 4.)Perfluoroalkyl derivatives such as the Surflon series (S383, S393, andS8405) from the Asahi Glass Company. In the method of the presentinvention, surfactants are generally of the non-ionic type. In apreferred embodiment of the present invention, non-ionic compounds ofeither the siloxane or fluorinated type are added to the uppermostlayers.

In terms of surfactant distribution, surfactants are most effective whenpresent in the uppermost layers of the multi-layer coating. In theuppermost layer, the concentration of surfactant is preferably0.001-1.000% by weight and most preferably 0.010-0.500%. In addition,lesser amounts of surfactant may be used in the second uppermost layerto minimize diffusion of surfactant into the lowermost layers. Theconcentration of surfactant in the second uppermost layer is preferably0.000-0.200% by weight and most preferably between 0.000-0.100% byweight. Because surfactants are only necessary in the uppermost layers,the overall amount of surfactant remaining in the final dried film issmall. In the method of the present invention, a practical surfactantconcentration in the uppermost layer having a wet thickness of 20 μm anda density of 0.93 g/cc is 0.200% by weight which after drying gives afinal surfactant amount of approximately 37 mg/sq-m.

Although surfactants are not required to practice the method of thecurrent invention, surfactants do improve the uniformity of the coatedfilm. In particular, mottle nonuniformities are reduced by the use ofsurfactants. In transparent cellulose acetate films, mottlenonuniformities are not readily visualized during casual inspection. Tovisualize mottle artifacts, organic dyes may be added to the uppermostlayer to add color to the coated film. For these dyed films,nonuniformities are easy to see and quantify. In this way, effectivesurfactant types and levels may be selected for optimum film uniformity.

Turning next to FIGS. 4 through 7, there are presented cross-sectionalillustrations showing various film configurations prepared by the methodof the present invention. In FIG. 4, a single-layer cellulose acetatefilm 150 is shown partially peeled from a carrier substrate 152.Cellulose acetate film 150 may be formed either by applying a singleliquid layer to the carrier substrate 152 or by applying a multiplelayer composite having a composition that is substantially the sameamong the layers. Alternatively in FIG. 5, the carrier substrate 154 mayhave been pretreated with a subbing layer 156 that modifies the adhesiveforce between the single layer cellulose acetate film 158 and thesubstrate 154. FIG. 6 illustrates a multiple layer film 160 that iscomprised of four compositionally discrete layers including a lowermostlayer 162 nearest to the carrier support 170, two intermediate layers164, 166, and an uppermost layer 168. FIG. 6 also shows that the entiremultiple layer composite 160 may be peeled from the carrier substrate170. FIG. 7 shows a multiple layer composite film 172 comprising alowermost layer 174 nearest to the carrier substrate 182, twointermediate layers 176, 178, and an uppermost layer 180 being peeledfrom the carrier substrate 182. The carrier substrate 182 has beentreated with a subbing layer 184 to modify the adhesion between thecomposite film 172 and substrate 182. Subbing layer 184 may be comprisedof a number of polymeric materials such as polyvinylbutyrals,cellulosics, polyacrylates, polycarbonates andpoly(acrylonitrile-co-vinylidene chloride-co-acrylic acid). The choiceof materials used in the subbing layer may be optimized empirically bythose skilled in the art.

The method of the present invention may also include the step of coatingover a previously prepared composite of cellulose acetate film andcarrier substrate. For example, the coating and drying system 10 shownin FIGS. 1 and 2 may be used to apply a second multi-layer film to anexisting cellulose acetate film/substrate composite. If thefilm/substrate composite is wound into rolls before applying thesubsequent coating, the process is called a multi-pass coatingoperation. If coating and drying operations are carried out sequentiallyon a machine with multiple coating stations and drying ovens, then theprocess is called a tandem coating operation. In this way, thick filmsmay be prepared at high line speeds without the problems associated withthe removal of large amounts of solvent from a very thick wet film.Moreover, the practice of multi-pass or tandem coating also has theadvantage of minimizing other artifacts such as streak severity, mottleseverity, and overall film nonuniformity.

The practice of tandem coating or multi-pass coating requires someminimal level of adhesion between the first-pass film and the carriersubstrate. In some cases, film/substrate composites having poor adhesionare observed to blister after application of a second or third wetcoating in a multi-pass operation. To avoid blister defects, adhesionmust be greater than 0.3 N/m between the first-pass cellulose acetatefilm and the carrier substrate. This level of adhesion may be attainedby a variety of web treatments including various subbing layers andvarious electronic discharge treatments. However, excessive adhesionbetween the applied film and substrate is also undesirable since thefilm may be damaged during subsequent peeling operations. In particular,film/substrate composites having an adhesive force of greater than 250N/m have been found to peel poorly. Films peeled from such excessivelywell-adhered composites exhibit defects due to tearing of the filmand/or due to cohesive failure within the film. In a preferredembodiment of the present invention, the adhesion between the celluloseacetate film and the carrier substrate is less than 250 N/m. Mostpreferably, the adhesion between polycarbonate film and the carriersubstrate is between 0.5 and 25 N/m.

The method of the present invention is suitable for application of resincoatings to a variety of substrates such as polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polycarbonate, polystyrene, andother polymeric films. Additional substrates may include paper,laminates of paper and polymeric films, glass, cloth, aluminum and othermetal supports. In some cases, substrates may be pretreated with subbinglayers or electrical discharge devices. Substrates may also bepretreated with functional layers containing various binders andaddenda.

The prior art method of casting resin films is illustrated in FIG. 8. Asshown in FIG. 8, a viscous polymeric dope is delivered through a feedline 200 to an extrusion hopper 202 from a pressurized tank 204 by apump 206. The dope is cast onto a highly polished metal drum 208 locatedwithin a first drying section 210 of the drying oven 212. The cast film214 is allowed to partially dry on the moving drum 208 and is thenpeeled from the drum 208. The cast film 214 is then conveyed to a finaldrying section 216 to remove the remaining solvent. The final dried film218 is then wound into rolls at a wind-up station 220. The prior artcast film typically has a thickness in the range of from 40 to 200 μm.

Coating methods are distinguished from casting methods by the processsteps necessary for each technology. These process steps in turn affecta number of tangibles such as fluid viscosity, converting aids,substrates, and hardware that are unique to each method. In general,coating methods involve application of dilute low viscosity liquids tothin flexible substrates, evaporating the solvent in a drying oven, andwinding the dried film/substrate composite into rolls. In contrast,casting methods involve applying a concentrated viscous dope to a highlypolished metal drum or band, partially drying the wet film on the metalsubstrate, stripping the partially dried film from the substrate,removing additional solvent from the partially dried film in a dryingoven, and winding the dried film into rolls. In terms of viscosity,coating methods require very low viscosity liquids of less than 5,000cp. In the practice of the method of the present invention the viscosityof the coated liquids will generally be less than 2000 cp and most oftenless than 1500 cp. Moreover, in the method of the present invention theviscosity of the lowermost layer is preferred to be less than 200 cp.and most preferably less than 100 cp. for high speed coatingapplication. In contrast, casting methods require highly concentrateddopes with viscosity on the order of 10,000-100,000 cp for practicaloperating speeds. In terms of converting aids, coating methods generallyinvolve the use of surfactants as converting aids to control flow aftercoating artifacts such as mottle, repellencies, orange peel, and edgewithdraw. In contrast, casting methods do not require surfactants.Instead, converting aids are only used to assist in the strippingoperation in casting methods. For example, n-butanol is sometimes usedas a converting aid in casting cellulose acetate films to facilitatestripping of the cellulose acetate film from the metal drum. In terms ofsubstrates, coating methods generally utilize thin (10-250 μm) flexiblesupports. In contrast, casting methods employ thick (1-100 mm),continuous, highly polished metal drums or rigid bands. As a result ofthese differences in process steps, the hardware used in coating isconspicuously different from those used in casting as can be seen by acomparison of the schematics shown in FIGS. 1 and 8, respectively.

The advantages of the present invention are demonstrated by thefollowing practical examples given below. In these examples, thecellulose acetate polymer was the highly substituted type (cellulosetriacetate with approximately 58.0-62.5% acetic acid content) with aweight average molecular weight of 228,000 daltons.

EXAMPLE 1

This example describes the single pass formation of an ultra thincellulose acetate film. The coating apparatus 16 illustrated in FIG. 1was used to apply four liquid layers to a moving substrate 12, 170 ofuntreated polyethylene terephthalate (PET) to form a single layer filmas illustrated earlier in FIG. 6. The substrate speed was 25 cm/s. Allcoating fluids were comprised of CTA dissolved in a blend of twosolvents (a 1:1 ratio of 1,3-dioxolane:acetone where the ratio is byweight). The lowermost layer 162 had a viscosity of 60 cp. and a wetthickness of 11 μm on the moving substrate 170. The second 164 and third166 layers each had a viscosity of 860 cp. and bad a combined final wetthickness of 69 μm on the moving substrate 170. The second 164 and third166 layers also each contained plasticizers (diethyl phthalate) at aconcentration of 15% by weight relative to the amount of CTA polymer. Inaddition, the third layer 166 also contained a fluorinated surfactant(Surflon S8405) at concentration of 0.02%. The uppermost layer 168 had aviscosity of 113 cp. and a wet thickness of 22 μm on the movingsubstrate 170. The uppermost layer 168 also contained a fluorinatedsurfactant (Surflon S8405) at a weight percent of 0.20%. Coatings wereapplied at a temperature of 24° C. The gap between the coating lip 136and the moving substrate 12 (see FIG. 3) was 200 μm. The pressuredifferential across the coating bead 146 was adjusted between 0-10 cm ofwater to establish a uniform coating. The temperature in the dryingsections 66 and 68 was 40° C. The temperature in the drying section 70was 50° C. The temperature in the drying sections 72, 74, 76, 78, 80 was95° C. The temperature in the drying section 82 was 25° C. The compositeof CTA film and PET substrate was wound into rolls. When peeled from theuntreated PET substrate, the final dry film had a thickness of 5 μm. Thepeeled CTA film had a good appearance, was smooth, was free fromwrinkles and cockle artifacts, and had an in-plane retardation of lessthan 1.0 nm. Properties of this cellulose acetate film are summarized inTable I.

EXAMPLE 2

This example describes the single pass formation of a very thincellulose acetate film. The conditions were identical to those describedin Example 1 except that the combined wet thickness of the second andthird layers 164 and 166 was increased to 154 μm. The composite of CTAfilm and PET substrate was wound into rolls. When peeled from the subbedPET substrate, the final dry film had a thickness of 10 μm. The peeledCTA film had a good appearance, was smooth, was free from wrinkles andcockle artifacts, and had an in-plane retardation of less than 1.0 nm.Properties of this cellulose acetate film are summarized in Table I.

EXAMPLE 3

This example describes the single pass formation of a thin celluloseacetate film. The conditions were identical to those described inExample 1 except that the combined wet thickness of the second and thirdlayers 164 and 166 was increased to 324 μm. The composite of CTA filmand PET substrate was wound into rolls. When peeled from the subbed PETsubstrate, the final dry film had a thickness of 20 μm. The CTA film hada good appearance, was smooth, was free from wrinkles and cockleartifacts, and had an in-plane retardation of less than 1.0 nm.Properties of this cellulose acetate film are summarized in Table I.

EXAMPLE 4

This example describes the single pass formation a cellulose acetatefilm. The conditions were identical to those described in Example 3except that the combined wet thickness of the second and third layers164 and 166 was increased to 665 μm. The composite of CTA film and PETsubstrate was wound into rolls. When peeled from the subbed PETsubstrate, the final dry film had a thickness of 50 μm. The peeled CTAfilm had a good appearance, was smooth, was free from wrinkles andcockle artifacts, and had an in-plane retardation of less than 1.0 nm.Properties of this cellulose acetate film are summarized in Table I.

EXAMPLE 5

This example describes the formation of a cellulose acetate film using atwo-pass coating operation. The conditions were identical to thosedescribed in Example 1 except that the solvent system was changed to a9:1 ratio of methylene chloride:methanol where ratios are by weight. Inaddition, the lowermost layer 162 had a viscosity of 66 cp. and a wetthickness of 11 μm on the moving substrate 170. The second 164 and third166 layers each had a viscosity of 1600 cp. and had a combined final wetthickness of 199 μm on the moving substrate 170. The second 164 andthird 166 layers each contained plasticizers (diethyl phthalate) at aconcentration of 20% by weight relative to the amount of celluloseacetate polymer. In addition, the third layer 166 also contained afluorinated surfactant (FC431) at concentration of 0.05%. The uppermostlayer 168 had a viscosity of 111 cp. and a wet thickness of 22 μm on themoving substrate 170. The uppermost 1 layer 168 also contained afluorinated surfactant (FC431) at a weight percent of 0.25%. Thecomposite of CTA film and PET substrate was wound into rolls. The woundcomposite was then over-coated with the combined wet thickness of thesecond and third layers at 199 μm. The composite of CTA film and PETsubstrate was wound into rolls. The final dry film had a thickness of 40μm. The peeled CTA film was smooth, was free from wrinkles and cockleartifacts, and had an in-plane retardation of less than 1.0 nm.Properties of this cellulose acetate film are summarized in Table I.

EXAMPLE 6

This example describes the formation of a cellulose acetate film using athree-pass coating operation. The conditions were identical to thosedescribed in Example 5 except that the wound composite of CTA film andPET support of Example 5 was over-coated with a third and final pass.For this final overcoat, the second 164 and third 166 layers had acombined final wet thickness of 411 μm on the moving substrate 170. Thecomposite of CTA film and PET substrate was wound into rolls. The finaldry film had a thickness of 80 μm. The peeled CTA film was smooth, wasfree from wrinkles and cockle artifacts, and had an in-plane retardationof less than 1.0 nm. Properties of this cellulose acetate film aresummarized in Table I.

COMPARATIVE EXAMPLE 1

This example describes defects formed in a cellulose acetate film as aresult of poor drying conditions during a single pass operation. Theconditions for Comparative Example 1 were identical to those describedin Example 5 except that the drying conditions were adjusted such thatthe temperature in the first three drying zones 66, 68, 70 weredecreased to 25° C. In addition, the CTA film was formed with only onepass through the coating machine. When peeled from the subbed PETsubstrate, the final dry film had a thickness of 20 μm. The peeledcellulose acetate film was of unacceptable quality due to fogging of thefilm.

COMPARATIVE EXAMPLE 2

This example describes defects formed as a result of poor dryingconditions during a single pass operation. The conditions forComparative Example 2 were identical to those described in ComparativeExample 1 except that the drying conditions were adjusted such that thetemperature in the first three drying zones 66, 68, 70 was increased to95° C. When peeled from the subbed PET substrate, the final dry film hada thickness of 20 μm. The peeled cellulose acetate film was ofunacceptable quality due to a reticulation pattern in the film as wellas to blister artifacts.

COMPARATIVE EXAMPLE 3

This example describes film defects in a single-pass coating operationcaused by inappropriate amounts of plasticizers. Comparative Example 3encompasses 25 different coating experiments. The conditions forComparative Example 3 were identical to those described in Example 5except that the level of plasticizers in each of the second 164 andthird 166 layers was adjusted to be 0, 5, 10, 20 and 30% diethylphthalate where the percentage is with respect to the amount of CTApolymer. In addition the wet coverage of the second and third layers wasadjusted to produce films having a final dry thickness of 5, 10, 20, 30and 40 μm in a single-pass operation. For very thin coatings of 5 and 10μm, dried film samples were of good quality regardless of the level ofplasticizers. Thicker cellulose acetate films of 20, 30 and 40 Jim,however, prematurely delaminated from the PET support during the dryingoperation when no plasticizers were used in the second and third layers.Premature delamination was avoided for 20, 30 and 40 μm CTA films by theuse of plasticizers at any level of 5, 10, 20 or 30%. At a level of 30%plasticizers, however, dried CTA films exhibited unacceptable bleedingof plasticizers after several days at ambient conditions. At a level of30 % plasticizers, this bleeding phenomenon is easily recognized duringvisual inspection of a dried CTA film by the presence of small oilydroplets on the surface. Thus, there appears to be an optimum level ofplasticizers in the range of 0 to 30% by weight with respect to theamount of cellulose acetate polymer.

COMPARATIVE EXAMPLE 4

This example describes the effect of surfactants on the quality ofcellulose acetate films in a single-pass coating operation. Theconditions for Comparative Example 4 were identical to those describedearlier in Examples 2 and 3 except that the type of surfactant used inthe uppermost layer 168 was altered. For these experiments, thesurfactant used in the uppermost layer included either a fluorinatedcompound (Surflon S8405, FC431, or Zonyl FSN), a siloxane compound(DC190), or none as noted. When present, surfactants were used at alevel of 0.20% by weight. In addition, a dye (Sudan Black) was added tothe uppermost layer 168 at a concentration of 1% by weight in order tohighlight nonuniformities in the final cellulose acetate film. The finaldyed CTA films had a dry thickness of either 10 or 20 μm as noted. Allsamples prepared with Zonyl FSN as the surfactant were grossly defectivedue to repellency artifacts. The remaining films did not exhibit grossnonuniformities and were evaluated for mottle nonuniformity bymeasurement of a Mottle Index for each sample. The Mottle Index is aquantitative measure of mottle nonuniformity with higher valuesindicating more severe nonuniformity. FIG. 9 summarizes the results. Asshown in FIG. 9, thicker films of 20 μm generally have higher levels ofmottle nonuniformity when compared to thinner films of only 10 μm. Forexample, 10 and 20 μm CTA films prepared without surfactant have aMottle Index of 277 and 465, respectively. Surfactants were found toreduce mottle severity, and fluorinated surfactants were most effective.For example, 10 and 20 μm cellulose acetate films prepared with thefluorinated surfactant, Surflon S8405, have a Mottle Index of only 172and 216, respectively. Thus, surfactants are shown here to substantiallyimprove the overall uniformity of the coated cellulose acetate films.This example illustrates an optimization process for choosing aneffective surfactant for the method of the present invention. This samestrategy may also be employed to select optimum levels of surfactant.

TABLE I Example Thickness Retardation Transmittance Haze Roughness 1  5μm 0.1 nm 94.7% 0.3% 2.0 nm 2 10 0.1 94.8 0.4 1.1 3 20 0.1 94.7 0.3 0.64 50 0.2 94.1 0.6 1.3 5 40 0.2 94.6 0.4 1.2 6 80 0.1 94.3 0.6 1.3

The following tests were used to determine the film properties given inTable 1.

Thickness. Thickness of the final peeled film was measured in micronsusing a Model EG-225 gauge from the Ono Sokki Company.

Retardation. In-plane retardation (R_(e)) of peeled films weredetermined in nanometers (nm) using a Woollam M-2000V SpectroscopicEllipsometer at wavelengths from 370 to 1000 nm. In-plane retardationvalues in Table I are computed for measurements taken at 590 nm.In-plane retardation is defined by the formula:R _(e) =|n _(x) −n _(y) |×dwhere R_(e) is the in-plane retardation at 590 nm, n, is the index ofrefraction of the peeled film in the slow axis direction, n_(y) is theis the index of refraction of the peeled film in the fast axisdirection, and d is the thickness of the peeled film in nanometers (nm).Thus, R_(e) is the absolute value of the difference in birefringencebetween the slow axis direction and the fast axis direction in the planeof the peeled film multiplied by the thickness of the film.

Transmittance and Haze. Total transmittance and haze are measured usingthe Haze-Gard Plus (Model HB-4725) from BYK-Gardner. Total transmittanceis all the light energy transmitted through the film as absorbed on anintegrating sphere. Transmitted haze is all light energy scatteredbeyond 2.5° as absorbed on an integrating sphere.

Mottle Index. Mottle levels are characterized in dyed samples using theMTI Mottle Tester from Tobias Associates. This device is a reflectancedensitometer with the capability of scanning 500 sample areas (1.5 mm indiameter) per scan. Twenty scans are performed for each sample. Samplesize is 5×20 cm. For these samples, the measured Mottle Index isconsistent with visual observations of the films.

Surface Roughness. Surface roughness was determined in nanometers (nm)by scanning probe microscopy using TappingMode™ Atomic Force Microscopy(Model D300 from Digital Instruments).

Adhesion. The adhesion strength of the coated samples was measured inNewtons per meter (N/m) using a modified 180° peel test with an Instron1122 Tensile Tester with a 500 gram load cell. First, 0.0254 m (oneinch) wide strips of the coated sample were prepared. Delamination ofthe coating at one end was initiated using a piece of 3M Magic Tape. Anadditional piece of tape was then attached to the delaminated part ofthe coating and served as the gripping point for testing. The extendingtape was long enough to extend beyond the support such that the Instrongrips did not interfere with the testing. The sample was then mountedinto the Instron 1122 Tensile Tester with the substrate clamped tin theupper grip and the coating/tape assembly clamped in the bottom grip. Theaverage force (in units of Newtons) required to peel the coating off thesubstrate at a 180° angle at speed of 2 inches/min (50.8 mm/min) wasrecorded. Using this force value the adhesive strength in units of N/mwas calculated using the equation:S _(A) =F _(p)(1−cos θ)/wwherein S_(A) is the adhesive strength, F_(p) is the peel force, θ isthe angle of peel (180°), and w is the width of the sample (0.0254 m).

Residual Solvent A qualitative assessment of residual solvents remainingin a dried film is done by first peeling the film from the carriersubstrate, weighing the peeled film, incubating the film in an oven at100° C. for 16 hours, and finally weighing the incubated film. Residualsolvent is expressed as percentage of the weight difference divided bythe post-incubation weight.

From the foregoing, it will be seen that this invention is one welladapted to obtain all of the ends and objects hereinabove set forthtogether with other advantages which are apparent and which are inherentto the apparatus.

It will be understood that certain features and sub-combinations are ofutility and may be employed with reference to other features andsub-combinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth and shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

PARTS LIST: 10 drying system 12 moving substrate/web 14 dryer 16 coatingapparatus 18 unwinding station 20 back-up roller 22 coated web 24 dryfilm 26 wind up station 28 coating supply vessel 30 coating supplyvessel 32 coating supply vessel 34 coating supply vessel 36 pumps 38pumps 40 pumps 42 pumps 44 conduits 46 conduits 48 conduits 50 conduits52 discharge device 54 polar charge assist device 56 opposing rollers 58opposing rollers 60 cellulose acetate film 62 winding station 64 windingstation 66 drying section 68 drying section 70 drying section 72 dryingsection 74 drying section 76 drying section 78 drying section 80 dryingsection 82 drying section 92 front section 94 second section 96 thirdsection 98 fourth section 100 back plate 102 inlet 104 metering slot 106pump 108 lower most layer 110 inlet 112 2^(nd) metering slot 114 pump116 layer 118 inlet 120 metering slot 122 pump 124 form layer 126 inlet128 metering slot 130 pump 132 layer 134 incline slide surface 136coating lip 138 2^(nd) incline slide surface 140 3^(rd) incline slidesurface 142 4^(th) incline slide surface 144 back land surface 146coating bead 150 cellulose acetate film 152 carrier substrate 154carrier substrate 156 subbing layer 158 cellulose acetate film 160multiple layer film 162 lower most layer 164 intermediate layers 166intermediate layers 168 upper most layer 170 carrier support 172composite film 174 lower most layer 176 intermediate layers 178intermediate layers 180 upper most layers 182 carrier substrate 184subbing layer 200 feed line 202 extrusion hopper 204 pressurized tank206 pump 208 metal drum 210 drying section 212 drying oven 214 cast film216 final drying section 218 final dried film 220 wind up station

1. A composite film comprising: a cellulose acetate film coated on adiscontinuous carrier substrate, the cellulose acetate film having athickness in the range of from about 1 to about 500 μm, the celluloseacetate film having an in-plane retardation that is less than 10 nm, thecellulose acetate film being adhered to the carrier substrate with anadhesive strength of less than about 250 N/m.
 2. A composite film asrecited in claim 1 wherein: the cellulose acetate film has an in-planeretardation that is less than 5 nm.
 3. A composite film as recited inclaim 1 wherein: the cellulose acetate film has an in-plane retardationthat is less than 1.0 nm.
 4. A composite film as recited in claim 1wherein: the cellulose acetate film is adhered to the carrier substratewith an adhesive strength of at least about 0.3 N/m.
 5. A composite filmas recited in claim 1 wherein: the cellulose acetate film is peelablefrom the carrier substrate.
 6. A composite film as recited in claim 1wherein: the cellulose acetate film is a multi-layer composite.
 7. Acomposite film as recited in claim 6 wherein: at least a top layer ofthe multi-layer composite includes a surfactant therein.
 8. A compositefilm as recited in claim 1 wherein: a plasticizer is incorporated in thecellulose acetate film.
 9. A composite film as recited in claim 1wherein: the cellulose acetate film has a light transmittance of atleast about 85 percent and a haze value of less than about 1.0 percent.